1. Field of the Invention
This invention relates to a process and apparatus for fluid catalytic cracking a hydrocarbon feed utilizing a mixture of large pore zeolite cracking catalyst particles and intermediate pore zeolite cracking catalyst particles. More particularly, the present invention relates to a process and apparatus which separates more elutriatable particles, which are predominantly intermediate pore zeolites, from less elutriatable particles, which are predominantly large pore zeolites.
2. Discussion of the Prior Art
In known and conventional catalytic cracking processes, a hydrocarbon feedstock, such as gas oil, is cracked in an elongated reactor riser, at elevated temperature, to provide a mixture of lighter hydrocarbon products. Suitable cracking catalysts include large pore crystalline zeolites, such as zeolite X or Y, and intermediate pore crystalline zeolites, such as ZSM-5. The products of the reaction, together with catalysts, are discharged into a separator located within an enclosed stripping vessel, with the spent catalyst flowing downwardly therein. Prior to transfer to a catalyst regenerator unit, entrained hydrocarbon product is removed from the spent catalyst by means of a stripping gas, such as steam or nitrogen. Following regeneration, the catalyst is reintroduced into the riser to participate in another cycle of operation. Fluid catalytic processes are described in U.S. Pat. Nos. 3,617,497 to Bryson et al and 4,219,407 to Haddad et al.
It is known to catalytically crack hydrocarbons with mixtures of ZSM-5 and other zeolites, as disclosed in U.S. Pat. No. 3,758,403 to Rosinski et al and incorporated herein by reference. Rosinski et al treat the ZSM-5 and the other catalysts equally by sending all the catalyst from a reactor to a regenerator. Thus, Rosinski et al do not recognize or take advantage of differences between ZSM-5 and other zeolites.
It would be desirable to provide a process and apparatus which employs large pore zeolite catalysts and intermediate pore zeolite catalysts and regenerates large pore zeolite catalysts more than intermediate pore zeolite catalysts. This is due to the fact that intermediate pore zeolite catalysts do not deactivate with coke formation as rapidly as large pore zeolites.
By segregating the intermediate pore zeolite catalysts, they are exposed to less hydrothermal degradation of catalyst when hydrogen-containing components, such as coke, adhering to catalyst passing from the fluid catalystic cracking stripper to the fluid catalystic cracking regenerator, react with oxygen in the regenerator to form water. This will result in less intermediate pore zeolite catalyst makeup required to achieve the desired results.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, Dean et al send all the catalyst from a reactor to a regenerator, thus providing opportunity for hydrothermal degradation.
It would be desirable to provide a process and apparatus which avoids sending a catalyst, such as intermediate pore zeolite catalysts, unnecessarily to the regenerator. In most instances, the intermediate pore zeolite catalyst does not deactivate with the amount of coke on it from one pass through the riser. Sending this partially-coked, but still highly active, intermediate pore zeolite catalyst unnecessarily to the regenerator results in unnecessary hydrothermal degradation of the intermediate pore zeolite catalyst in the regenerator.